This invention relates to a method for combusting fuels which include nitrogen-containing compounds so that formation of nitrogen oxides (NO.sub.x) from such compounds, which ordinarily tends to occur during combustion, is suppressed materially.
In general, nitrogen oxides are formed as by-products of combustion processes carried out with air at relatively high temperatures. As used herein and in the appended claims, the term air means any gas or combination of gases containing oxygen available for combustion reactions and also containing ordinarily inert materials including nitrogen gas. The term stoichiometric amount of air means that amount of air which is theoretically sufficient for complete oxidation of all the combustible components in a given amount of fuel (e.g., to carbon dioxide and water). Particularly in combustors used in furnaces, boilers, process drying equipment, and gas turbines, in which peak combustion temperatures typically exceed about 3,200.degree. F, atmospheric nitrogen in the feed to the combustors is oxidized to produce relatively large amounts of nitrogen oxides. As a result, the conventional high temperature combustors used for producing heat and power in modern technology have tended to cause the accumulation of nitrogen oxides in the atmosphere. In fact, the discharge of nitrogen oxides from various sources has become an environmental hazard, especially in urban areas. For this reason, governmental agencies are concerned with more or less stringent nitrogen oxide emission standards for all combustion equipment.
The difficulties in minimizing nitrogen oxide emissions have been aggravated by the energy crisis. This has resulted from diminished supplies of relatively clean-burning hydrocarbon fuels, e.g., natural gas, which has made the use of so-called "dirty" fuels more attractive or even a necessity. The "dirty" fuels, such as coal gas, number 6 diesel fuel, shale oil, and of course coal-derived liquid fuels, have typically contained, as impurities, sizable amounts of fuel nitrogen, i.e., nitrogen-containing compounds, as for example ammonia in coal gas, and cyclic and polycyclic nitrogen compounds, e.g., compounds in the carbazole, pyridine, indole, and aniline families, in some liquid fuels. In combustors generally, a substantial portion of the fuel nitrogen in "dirty" fuels is oxidized and converted to nitrogen oxides. The combination of the oxidation of atmospheric nitrogen and the oxidation of nitrogen-containing compounds originating in fuels has tended to produce undesirably high nitrogen oxide levels in the effluents of conventional, high temperature combustors, burning "dirty" fuels. Hence efficient combustion methods have been sought in which the oxidation of nitrogen-containing compounds in "dirty" fuels to nitrogen oxides is inhibited and, at the same time, the formation of nitrogen oxides from atmospheric nitrogen is inhibited or substantially avoided.
One proposal for minimizing such formation of nitrogen oxides involves operating a fire tube boiler with combustion of the fuel in two stages, the boiler being extended somewhat to provide two axially aligned combustion chambers. (Paper by D. W. Turner and C. W. Siegmund, "Staged Combustion and Flue Gas Recycle: Potential for Minimizing NO.sub.x from Fuel Oil Combustion", presented at The American Flame Research Committee Flame Days, Chicago, Ill., Sept. 6-7, 1972). To aid in limiting total formation of nitrogen oxides from nitrogen-containing compounds in the fuel as well as from atmospheric nitrogen in the combustion air, it was proposed to operate the first stage moderately fuel-rich; some excess air is added to the partially combusted effluent, and the remaining uncombusted fuel is burned in the second stage. The modified boiler was tested by progressively decreasing the rate of air supply to the first stage relative to the rate of fuel feed. As the air supply rate is decreased from a little excess air, through the stoichiometric amount, and somewhat into the fuel-rich region, the total amount of nitrogen oxides formed decreases although combustion zone temperatures remain high. As the feed is made still more fuel-rich, nitrogen oxide formation continues to decrease. However, as this occurs, combustion zone temperatures also decrease more and more sharply, and the combustion in the first stage becomes increasingly unstable as the operating region is approached (at an amount of air equal to about 0.8 to 0.7 times that needed for complete combustion) where the largest decreases in total nitrogen oxide formation are achieved in spite of the presence of substantial amounts of nitrogen-containing compounds in the fuel. Thus, to realize the benefits of desirably low nitrogen oxide formation, it becomes necessary to sacrifice combustion stability and dependability or to maintain stability by other means, such as vigorous circulation within the combustion zone, or sharp limitation of the space velocity of the fuel-air mixture passed through the combustion zone. Unfortunately, the alternative of operating at higher air-fuel ratios in order to improve combustor stability results in rather sharp increases of total nitrogen oxides formed. Accordingly, a method of achieving combustion with dependable stability, even at high throughput rates, and without excessive total formation of nitrogen oxides from fuel nitrogen as well as atmospheric nitrogen, would be useful and desirable.
A particularly attractive method for avoiding substantial formation of nitrogen oxides from atmospheric nitrogen in the combustion of fuels to generate heat and power has been disclosed in U.S. patent application Ser. No. 358,411, filed May 8, 1973, in the name of William C. Pfefferle and assigned to the same assignee as that of the present invention, entitled "Catalytically Supported Thermal Combustion", now U.S. Pat. No. 3,928,961, which is incorporated by reference in the present application. The method of this earlier application, employing a catalyst operating under specified conditions in the combustion zone, may be used advantageously in carrying out a preferred embodiment of the method of the present invention. Another U.S. patent application of William C. Pfefferle, Ser. No. 519,288, filed Oct. 30, 1974, entitled "Method and Apparatus for Turbine System Combustor Temperature Control", and also assigned now U.S. Pat. No. 3,975,900, to the same assignee as that of the present invention, discloses a method of controlling a combustor, which feeds a gas turbine, to maintain constant operating temperature of a catalyst in the combustion zone. This application mentions a number of fuels typically low in nitrogen-containing compounds, exemplified by commercial gasoline, naphtha, and propane, and describes combustion temperature control by automatic adjustments in the fuel-air mixtures which are chosen to remain sufficiently fuel-lean or fuel-rich to burn at temperatures of the order of 3,200.degree. F or lower in the presence of the catalyst. When fuel-rich mixtures are used in such a method, application Ser. No. 519,288 notes that the partially oxidized effluent can be mixed with additional air and thermally combusted downstream of the catalyst.